(a) Field of the Invention
The present invention relates to a liquid developing apparatus, more in detail to the liquid developing apparatus in which spread of a surplus developer concentrated on both ends of a photosensitive element is prevented, an efficiency of squeezing is elevated, and the surplus developer is removed.
(b) Description of the Related Art
In some of the conventional liquid developing apparatuses, a surplus developer is removed, and adhesion of toner on a photosensitive element, a developing roller and a squeezing roller is prevented in addition to acceleration of development by applying a bias voltage to a developing element and a squeezing element.
JP-A-9(1997)-15981 describes a surplus developer removing apparatus which employs a squeezing roller including a porous elastic member and a conductive surface layer made by a material having a low surface energy and high air-permeability formed thereon. In the surplus developer removing apparatus, adhesion of toner and lowering of a function of removing the surplus developer are prevented by applying, to the squeezing roller, a voltage between several hundreds to several thousands Volts having the same polarity as that of the toner.
JP-A-6(1994)-19266 describes a liquid developing apparatus which includes a first developing electrode for applying a bias voltage having the same polarity as that of all electrostatic latent image and a second developing electrode for applying a bias voltage having the reverse polarity. In the apparatus, by making the electrodes a dish-like shape, a longer and effective developing time can be secured, a development fogging is suppressed after removing the toner on the non-image portion, and the adhesion of the toner on the developing electrodes is prevented.
JP-A-56(1981)-81870 describes a roller to which a bias voltage having a polarity reverse to that of toner is applied is disposed between a developing part and a transcribing part and near a photosensitive element to remove a surplus developer in an image-forming process.
JP-A-10(1998)-274885 describes a liquid developing apparatus in which a bias voltage is applied between a developing roller and a cleaning roller not in contact with the developing roller to remove remaining toner on the developing roller.
It is also described in a publication that first and second squeezing rollers are used in a conventional liquid developing apparatus, as shown in FIG. 1. When a photosensitive element on which a developer is applied by a developing roller (not shown) moves from left to right in FIG. 1 and passes the first squeezing roller 11, the developer is squeezed from the edges of the first squeezing roller 11, and the surplus developer is likely to be left on the both edges of the sensitive element.
The second squeezing roller 12 having a pair of removing elements 13 on the both ends thereof is mounted immediately after the squeezing roller 11 to rotate in the direction reverse to the moving direction of the photosensitive element, thereby removing the surplus toner on the belt-like photosensitive element.
Another apparatus is proposed in which air nozzles are disposed on both sides of the moving photosensitive element and near the squeezing roller to remove the developer by preventing the flow-out of the surplus developer.
However, the conventional liquid developing apparatuses have the following problems.
Since the developer supplied to the photosensitive element by means of the developing roller spreads in the direction of the roller axis when a pressure is applied to the developer by the squeezing roller, the surplus developer is likely washed away and spreads towards the both ends of the photosensitive element to be concentrated thereon. When the conventional liquid developing apparatus is applied to a so-called one-pass type color printer, a surplus developer having a first color spreading to the both ends of the photosensitive element remains to be mixed with a developer having a second color, thereby lowering the reproducibility of the colors to deteriorate chroma.
In order to solve the problem, the removal of the surplus developer remaining on the both edges of the photosensitive element may be conducted by employing the secondary squeezing roller 12 as shown in FIG. 1. However, the toner adhered to the secondary squeezing roller 12 and the abrasion thereof gradually reduce the ability of removing the developer, and a stain on the image ends is generated by the developer remaining on the both edges of the photosensitive element.
In order to remove the surplus carrier by the air nozzles, an air pump having a larger capacity is required because an amount of the surplus developer is large to increase an amount of air-flow required therefor. The employment of the air pump having the larger capacity may be a cause of pollution and a noise generated by carrier components of the developer evaporated in the apparatus.
A number of proposals have been conventionally presented as described above in which the surplus carrier is removed and the adhesion of the toner is prevented by applying the bias voltage, and the squeezing efficiency of the squeezing roller is improved by removing the surplus carrier from the developing roller, the squeezing roller and the photosensitive element. However, the conventional techniques do not pay attention to the pollution of the image edges.